This invention relates to electroless plating methods and apparatus, and particularly to such methods and apparatus used for plating symmetrical plates.
Electroless plating apparatus typically must perform to highly exacting standards with respect to several critical parameters, and often the failure, by even a slight degree, to meet any of such standards may result in ruined workpieces or non-acceptable quality in the plated pieces.
The electroless plating process is an autocatalytic one, in which the surface of the workpiece, initially, and subsequently the built-up surface of the metal that is being deposited, provides a catalytic surface for the subsequent deposition reaction; consequently any masking of portions of the surface even temporarily, as by unwanted chemical products of the reactions (e.g., acids and precipitates) tends to slow down the deposition process at such points, producing a coating of uneven thickness.
To avoid this eventuality, the present invention contemplates agitating the bath, rotating the workpiece in a bath-agitating manner; and bubbling compressed air from bottom to top through the bath to "sparge" the workpiece surface, i.e., to sweep it clear of unwanted chemicals and to ensure continuous accessibility of the surface to fresh concentrations of bath constituents. In addition the bath is heated, as by immersion heaters or by other types of heaters, to accelerate the plating rate.
The heating action, however, produces a vertical temperature gradient in the solution, hot at the bottom, cooler at the top; also, getting the maximum number of workpieces into the solution requires placing some of the workpieces at higher or lower levels than others in the bath. Consequently, it is desirable that each workpiece be moved bodily and in sequence through both upper and lower portions of the bath, to achieve uniformity of plating action as between workpieces.
In developing the present invention for the nickel-plating of aluminum computer-memory magneticdisc body pieces, it was first attempted to mount the discs in sub-sets on central spindles, and to attach a number of the spindles to a vertically-rotating drum-like structure, so that each spindled sub-set was in turn rotated between top and bottom portions of the bath, as for deposition thereon of a smooth flat nickel surface (about 0.7 mils thick) suitable as a substrate for later deposition of an extremely thin (e.g., less than 5 micro-inch) coating of magnetic material.
However, plating by this method was found to produce, at certain edge portions of each disc, an extremely roughened nickel surface together with imperfect adhesion of the nickel strip to the aluminum base such that, during ordinary handling, bits of the plated nickel often became snagged on hands, gloves or other ambient objects and broke or flaked off; furthermore this edge flaking action often extended around to the face of the disc and tore off bits of the actual substrate upon which the thin magnetic layer had subsequently been plated.
Painstaking investigation eventually identified the cause of this phenomenon: it appears that the sparging bubbles have a tendency to adhere to the "leading edge" of each disc as it moves in its circular orbit around the central shaft; consequently at the site of each such bubble, the nickel deposit has a tiny hole or thin spot, which can be detected by dipping the edge into sulfuric acid, so as to initiate a visible reaction with the underlying aluminum. Conversely, the zones between bubbles may be plated more thickly than otherwise, but are insecurely attached to the aluminum body.
Accordingly, it is an object of the present invention to electrolessly plate disc-like workpieces without imperfect edge plating.
To meet this object the present invention contemplates mounting the workpieces on spindles in a heated and sparged bath, rotating each spindle not only about an eccentric axis, but also its own axis, so that bubbles adhering to the leading edges of the workpieces are quickly swept off, and the edge plating of the pieces is solid, smooth and uniform, with normal adhesion characteristics.